Process for fabricating stacked trench capacitors of dynamic ram
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for fabricating the stacked trench capacitor (STC) of a dynamic random access memory (DRAM), particularly to the process for fabricating the STC structures which are electrically isolated between trenches. 
     In the present invention, the side wall of the trench, which is encapsulated by a thermal oxide layer, is covered with a side-spacer silicon layer by deposition and subsequent anisotropic dry etch of the CVD silicon. Since the oxide film which is not protected by a side-spacer silicon layer is removed by wet etch, only the side wall of the trench is covered by an oxide/silicon bilayer. The oxide film serves as an insulating layer to eliminate the trench-to-trench leakage current. The electrical isolation is also improved by ion-implantating the dopants underneath the trenches. Further, since the inventive capacitor structure consists of a sloped shape of trench side wall and has no sharp edges, electrical weak spots of the capacitors can be eliminated. 
     2. Description of Related Art 
     In order to realize the stacked capacitors for a high density DRAM, a shallow trench is made into a silicon substrate to increase the capacitor area, hence increasing the capacitance. Conventional processing procedures are as follows: 
     First, as shown in FIG. 1 A, the CVD oxide 10 and poly silicon films 11 are sequentially deposited on the gate 1. As shown in FIG. 1 B, a trench 2 is made by opening the contact window and etching down into a silicon substrate 9 using reactive ion etch (RIE). A poly-silicon layer 4 which serves as a storage node is deposited by CVD, and doped using ion implantation or diffusion from dopant vapor source, as shown in FIG. 1 C. And then, a capacitor area is defined by photo and etch. Finally, as shown in FIG. 1 D, the thin dielectric 6 layer is formed on the storage node poly silicon. Finally, the entire capacitor area is covered by poly-silicon. Finally, the entire capacitor area is covered by poly-silicon 7 for the opposite electrode. In the conventional art, the subsequent high temperature processes after doping the poly-silicon layer 4 lead to out diffusion of dopant from the poly-silicon layer to silicon substrates 9 forming a diffused region 12 around the trench. Formation of diffused region in contact with drain junction increases the contact area between capacitor and the junction. 
     However, in the conventional art described above, the presence of the dopant diffused regions 12 around the trenches cause a significant leakage current between trenches as shown in FIG. 1 E, which is a cross sectional view taken along line A--A in FIG. 1 F. The problem of leakage current between trenches becomes more significant as the spacing of the trenches decreases and/or the depth of the trenches increases. Another drawback of this structure is that since a vertical shape of the trench is formed after deposition of poly-silicon layer 4, the thin dielectric layer 6 formed later has electrical weak spots at the sharp edges in the bottom part of the trench. Therefore, this capacitor structure causes leakage current between the capacitor electrodes, and hence degrading the reliability of the capacitor. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to provide a process method which overcomes the drawbacks of the conventional art described above. The present invention provides an improved method for reducing leakage currents between trenches n the conventional STC structure. The present invention also provides an improved method for enhancing the reliability of the thin dielectric layer in the STC structure. 
     SUMMARY OF THE INVENTION 
     In the present invention, the processing procedures of STC structure of a DRAM which reduces the trench-to-trench leakage current are described. The bare surface of the trenches between the transistor gates is covered by a thermal oxide layer. The bottom of the trench is ion-implanted with a type of dopants opposite to that of source/drain of the transistor (e.g., p-type dopants for N-MOS). The portion of side wall of trench below the junction is selectively covered by side-spacer silicon layer utilizing the anisotropic RIE of CVD silicon. The thermal oxide layer which is not protected by side-spacer silicon layer is wet etched off. On this trench structure, the capacitor is fabricated by depositing silicon layer (storage node), thin dielectric layer, and another silicon layer (opposite plate) sequentially. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The process of the invention will be more visually described using the accompanying drawing in which: 
     FIG. 1 A. through 1 D are cross-sectional views taken along with the line B--B in FIG. 1 F for explaining the processing sequence of the conventional art process. 
     FIG. 1 E is a cross sectional view taken along the line A--A in FIG. 1 F. 
     FIG. 1 F is a plane view of the conventional DRAM cell. 
     FIG. 2 A through 2 D are cross sectional views taken along the line B--B in FIG. 1 F for explaining the processing sequence of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The procedure of the process of the present invention will be described in detail, taking an example of fabricating STC structure with a N-MOS transitor*. 
     Referring to FIGS. 2 A through 2 D, the processing sequence of the present invention is described. 
     First, as shown in FIG. 2 A, the trench is formed between transistor electrodes using the photolithography and RIE technique. And then an oxide film 3, which is several hundred angstroms thick, is thermally grown in a diffusion furnace. 
     Next, the P-type dopants 5a are ion-implanted for providing an electrical isolation in the bottom of the trench 2. The dose and energy of the ion implantation are decided considering thickness of the oxide layer, doping level of the storage node silicon layer 5 deposited as shown in FIG. 2 B later and ion-implantation damages. 
     After the ion implantation, an undoped poly or amorphous silicon layer 4 is deposited to a thickness of 1500 to 3000 angstrom as shown in FIG. 2 C, the silicon layer 4 is anisotropically etched by RIE leaving only the side spacer silicon layer 4a covering the side wall of the trench 2 which is below the N+ junction region 8 of the transistor. 
     Subsequently, the thermal oxide film 3 is wet etched so that the exact portion of the side wall of the N+ junction region is opened. In this process, the oxide film 3a covering the side wall deeper than the junction 8 can be survived, since it is protected by side-spacer silicon layer 4a. The 1000-3000 angstrom thick silicon layer 5, which serves as a storage node of the capacitor, is deposited as shown in FIG. 2 D. The capacitor region is defined using conventional photo and etch. 
     And then, the thin dielectric layer 6 is deposited, and subsequently covered by poly-silicon 7 for the opposited plate. 
     The STC structure fabricated by the invented process described above has advantages over conventional one in following aspects. 
     (1) The presence of the oxide layer at the sidewall of the trenches prevents the leakage of current through the side walls of trenches. 
     (2) The silicon substrates 5a underneath the bottom of the trench 2 is doped with p-type dopant, which reduces the current leakage through the bottom of the trenches. 
     (3) Since the storage node silicon layer 5 has smooth concave-shaped corners around the bottom of the trench, the thin dielectric layer can be deposited with a uniform thickness and without electrical weak spots where the charges may leak. In this way, more reliable capacitors with a reduced current leakage between the electrodes can be fabricated. 
     Because of the described beneficial features, the present STC structure can be applied for a high-density DRAM above 4M where the trench is deeper and the spacings between trenches narrower. 
     While the described embodiment represents the preferred form of the present invention, it is to be understood that modifications of the present processes will occur, however, without departing from the spirit of the invention. The scope of the invention is therefore to be determined solely by the appended claims.